In strongly exothermal or endothermal reactions, such as catalytic cracking, dehydrogenation and the combustion of solid fuels, there are used fluid-bed reactors (fluid catalytic reactors or fluid-bed boilers). By “fluid-bed apparatus” is meant below an apparatus used in processes wherein a finely-divided catalyst powder or solid is distributed in, for example, a gas stream moving slowly upwards, wherein it causes the desired reactions and/or transports thermal energy.
One of the most commonly used fluid-bed apparatuses is the FCC apparatus, i.e. fluid catalytic cracking apparatus, the main components of which are a riser operating in the range of a rapid fluidizing flow, a large-volume reactor operating in a dilute suspension phase, and a regenerator operating in the fluid-bed range.
In an FCC unit, the solid particles and product gas of the solids suspension obtained from the riser and the large-volume reactor are separated from each other in cyclones the operation of which is based on the exploitation of centrifugal force. Usually cyclones are installed in series in the direction of the gas flow in order to improve the total separation degree, since the degree of separation of normal individual cyclones is poor with particles of less than 15 μm.
The cyclones may be by structure helical or spiral, in which case the solids suspension is introduced as a tangential stream into the cylindrical part of the cyclone and the catalyst separates from the gas by passing to the vicinity of the wall, the stream typically circulating 7–9 rotations in the cylindrical part of the cyclone and in the conical part constituting its continuation. There are also known axial cyclones wherein a gas traveling in a pipe is brought by means of a vane system into a rotary motion, whereupon the solids are driven under centrifugal force to the wall of the pipe, thus separating from the gas stream.
Axial-flow cyclones are described in GB patent publications 1 592 051 and 1 526 509. The axial-flow cyclone according to these publications has a tubular cyclone chamber at the upstream end of which there is an inlet for the stream to be treated and at the other end an outlet for purified gas. It is proposed that the said cyclones be used in combustion, diesel and jet engines, turbines or similar apparatuses requiring pure feed air.
Stricter air protection requirements and the lowering of pressure, carried out by means of turbines, of the FCC regeneration gas, in order to make the use of energy more efficient, set even stricter limitations than previously on the dust content in FCC. It is possible to improve the separation efficiency by reducing the diameter of the cyclone, but the number of cyclones has to be increased correspondingly. It can be stated in general that the separation of small particles requires a small-diameter cyclone. However, the manufacture of small cyclones increases the cost of investment of the apparatus, unless there is used a structure allowing mass production by using, for example, thin-sheet technology. Furthermore, the manufacture of small parts, for example the welding of small vanes, is a problem in conventional methods. Commercial multi-cyclone options (e.g. Shell's TSS) for their part require a large-volume pressure vessel. The structure of multi-cyclones becomes a problem when the direction in which the gas is directed is the same as that from which it comes.
The problems involved with conventional FCC units include, in addition to deficient separation capacity, also the erosion of the catalyst/solid and the structures. The problems are most commonly associated with the gas and solids/catalyst separation cyclones that constitute an essential part of the apparatus. To prevent wear, conventional cyclone structures have to be lined with a ceramic paste. The problems caused by erosion become emphasized when the diameter of the cyclone is reduced.
The object of the present invention is to eliminate the disadvantages associated with the state of the art and to provide a novel option for the separation of solids from gases.
The invention is based on the idea that a separation apparatus is used which comprises a plurality of normal-structured but relatively small-sized multi-inlet cyclones arranged in parallel, the cyclones together constituting a cyclone system, i.e. a configuration made up of a plurality of cyclones.
The separating of solids from a gas stream with the help of a cassette containing a plurality of cyclone units is previously known from CA patent 853 025, DE patents 1 004 463 and 1 058 343 and U.S. Pat. Nos. 2,806,551 and 3,448,563. DE patent publication 1 058 343 and CA patent publication 853 025 disclose a cassette made up of axial-cyclone units, wherein the stream is directed to the cyclone in the axial direction. Owing to the feed direction the stream arrives in the cyclone chamber as a relatively wide jet, in which case the particles have on average a moderately long distance to travel from the feed inlet to the wall. Structurally known apparatuses are partly quite complicated, and their degree of separation is not sufficient to take into account the stricter requirements mentioned above.
GB patent publication 545 624 describes a cyclone unit in which a stream containing particles (dust) enters the cylindrical pipes of the cyclone system from the side via special inlets, and the stream is brought into a rotary motion. The separated dust flows down along the pipe wall into a collection silo, whereas the gas stream turns, in the manner of a conventional cyclone separator, upwards to an outlet pipe, to be directed away from it. The operation of the cyclone is based on the reverse flow principle, which leads to a relatively complicated structure.